Brownian Ratchet Mechanism for Faithful Segregation of Low-Copy-Number Plasmids.
Brownian Ratchet Mechanism for Faithful Segregation of Low-Copy-Number Plasmids.
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DOI：
10.1016/j.bpj.2017.02.039
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发表时间：
2017-04-11
影响因子：
3.4
通讯作者：
Liu J
中科院分区：
文献类型：
--
作者：
Hu L;Vecchiarelli AG;Mizuuchi K;Neuman KC;Liu J
细菌质粒是染色体外DNA，为细菌生存提供选择性优势。质粒分配可以是非常稳健的。对于高拷贝数质粒，扩散确保两个子细胞在细胞分裂后继承质粒。相反，大多数低拷贝数质粒需要通过保守的三部分ParA型系统进行主动分配。帕拉是一种与染色体DNA结合的ATP酶; ParB是帕拉ATP酶的刺激物，并在着丝粒样位点parS特异性结合质粒。ParB刺激帕拉ATP酶会从细菌染色体上释放出帕拉，之后需要很长时间才能重新设置其DNA结合亲和力。我们以前在体外证明，帕拉系统可以利用这种生物化学的不对称定向货物运输。多个ParA-ParB键可以将parS包被的货物连接到DNA地毯上，它们可以作为布朗棘轮共同工作，指导货物持续移动，后面是ParA耗尽区。通过扩展这个模型，我们认为，一个类似的布朗棘轮机制概括了在体内观察到的积极分离的质粒motilities的全部范围。我们表明，质粒运动性被调谐为补充率的ParA-耗尽区逐步增加相对于货物的速度，从扩散到极点到极点振荡，局部偏移，最后，不动。当质粒复制时，子体在很大程度上显示出与其母体相似的运动性，除了当单焦点祖体局部扩散时，子焦点进行定向分离。我们表明，定向分离最大限度地提高了质粒分区的保真度。鉴于本地偏移和定向分离是最常见的模式，在体内的质粒运动，我们建议的ParA型分区系统的操作已经形成了高保真度的质粒分离的进化。
Bacterial plasmids are extrachromosomal DNA that provides selective advantages for bacterial survival. Plasmid partitioning can be remarkably robust. For high-copy-number plasmids, diffusion ensures that both daughter cells inherit plasmids after cell division. In contrast, most low-copy-number plasmids need to be actively partitioned by a conserved tripartite ParA-type system. ParA is an ATPase that binds to chromosomal DNA; ParB is the stimulator of the ParA ATPase and specifically binds to the plasmid at a centromere-like site, parS. ParB stimulation of the ParA ATPase releases ParA from the bacterial chromosome, after which it takes a long time to reset its DNA-binding affinity. We previously demonstrated in vitro that the ParA system can exploit this biochemical asymmetry for directed cargo transport. Multiple ParA-ParB bonds can bridge a parS-coated cargo to a DNA carpet, and they can work collectively as a Brownian ratchet that directs persistent cargo movement with a ParA-depletion zone trailing behind. By extending this model, we suggest that a similar Brownian ratchet mechanism recapitulates the full range of actively segregated plasmid motilities observed in vivo. We demonstrate that plasmid motility is tuned as the replenishment rate of the ParA-depletion zone progressively increases relative to the cargo speed, evolving from diffusion to pole-to-pole oscillation, local excursions, and, finally, immobility. When the plasmid replicates, the daughters largely display motilities similar to that of their mother, except that when the single-focus progenitor is locally excursive, the daughter foci undergo directed segregation. We show that directed segregation maximizes the fidelity of plasmid partition. Given that local excursion and directed segregation are the most commonly observed modes of plasmid motility in vivo, we suggest that the operation of the ParA-type partition system has been shaped by evolution for high fidelity of plasmid segregation.